Daffy almost certainly has the answer. This should fix the problem:
https://addons.mozilla.org/en-US/firefox/addon/adblock-plus/
However if you ever do find that you've got problems with Firefox, you don't need to follow the uninstall+reinstall route. Simply go to Tools > Add-ons > Extensions and note down what you want to keep. (That's because you'll need to reinstall them later). Then go to Help > Troubleshooting Information and click on 'Reset Firefox'. Reinstall your extensions and everything should be OK.